According to the Multiboot specification, mem_lower specifies the amount (size) of lower memory (conventional memory below 1 MB) measured in kilobytes (KB). On a standard x86 PC architecture, mem_lower is almost always 640 (representing 640 KB of conventional memory spanning from physical address 0x00000 up to 0xA0000).
Therefore, mbi.mem_lower « 10 calculates the upper boundary (the end address) of conventional memory in bytes
For a standard 640 KB system, 640 * 1024 evaluates to 0xA0000 (the address right where the VGA text buffer and BIOS ROM areas begin).
https://gemini.google.com/app/35692b897ec58f41
Technical Context: Memory Map Location
In real mode and legacy x86 system initialization, physical address space below 1 MB (Conventional Memory) is partitioned as follows:
**
0x00000–0x9FFFF**(0 to 640 KB): Conventional System RAM (Usable by OS/Programs).**
0xA0000–0xBFFFF**(640 KB to 768 KB): Video Display RAM & Adapters.**
0xA0000–0xAFFFF**(64 KB): VGA Frame Buffer (Graphics modes).**
0xB0000–0xB7FFF**(32 KB): MDA / Monochrome text mode memory.**
0xB8000–0xBFFFF**(32 KB): CGA / VGA Color Text Mode frame buffer (e.g., standard DOS console, early OS boot printouts).**
0xC0000–0xFFFFF**(768 KB to 1 MB): Video ROM BIOS, Option ROMs, and System BIOS.
How to Use 0xA0000
make run
